1. Field of the Invention
This invention relates to a method and apparatus for detecting degradation of a metal material. More particularly, this invention relates to a degradation measuring method and apparatus suitable for detecting embrittlement, due to high-temperature aging, of practical plant components of a metal material such as a ferritic stainless steel used in an environment of high temperatures such as that encountered in a chemical plant and a nuclear power plant.
2. Description of the Prior Art
Methods of measuring embrittlement of a metal material are disclosed in, for example, JP-A-54-61981 and JP-A-61-28859. According to one of the prior art methods disclosed in JP-A-54-61981, a weld, metal such as an austenitic stainless steel is determined to be brittle when the amount of .delta.-ferrite has decreased by more than 5% of its initial value. Also, according to the prior art method disclosed in JP-A-61-28859, change in a magnetic characteristic of a measuring object is measured to detect degradation of the measuring object.
Further, JP-A-56-168545 discloses a method in which a magnetic characteristic of a measuring object is utilized to monitor the metallographic structure of the measuring object, and JP-A-59-108970 discloses a method of detecting a magnetic characteristic of a measuring object.
As disclosed in JP-A-54-61981 cited above, it is already known that aging degradation occurs on a metal material, especially, a ferritic stainless steel, when the stainless steel is used for a long period of time at high temperatures. That is, at a relatively high temperature higher than about 600.degree. C., .sigma.-embrittlement attributable to precipitation of a .sigma.-phase occurs, while in a temperature range of from 400.degree. C. to 500.degree. C., so called 475.degree. C.-embrittlement occurs. However, this 475.degree. C.-embrittlement may occur even when the ferritic stainless steel is used for a long period of time in a temperature range lower than 400.degree. C. Therefore, it is necessary to give sufficient consideration to the use of practical plant components of the ferritic stainless steel at high temperatures. However, the tendency to become brittle at temperatures lower than 500.degree. C. was not taken into consideration in the prior art disclosure cited above, and the 475.degree. C.-embrittlement could not be detected.
Further, the initial amount of ferrite in practical welded parts differs depending on the position of the welds and tends to greatly fluctuate. Furthermore, because the number of welds in a practical plant is very large, it is difficult to monitor the initial amount of ferrite for each of the welds and all equipments. Thus, the prior art method disclosed in JP-A-54-61981 could not be put into use in a practical plant because it was not applicable to parts where the initial amount of ferrite was unknown.
On the other hand, an eddy current test method (referred to hereinafter as an ECT) is disclosed in, for example, JP-A-55-141653. According to this prior art method, an ECT value of a measuring object, which is, for example, an iron alloy, is compared with an ECT value of the measuring object measured before practical use or an ECT value of the same material as the material of the measuring object and subjected to heat treatment similar to initial heat treatment applied to the measuring object, and the degree of degradation of the iron alloy is decided depending on whether the result of comparison is positive or negative. However, a quantitative measurement or analysis could not be made because the decision is merely based on whether the result of comparison is positive or negative.
In the prior art method disclosed in JP-A-61-28859, it is necessary to measure the initial value of a magnetic characteristic of a measuring object. Also, the prior art methods disclosed in JP-A-59-108970 and JP-A-56-168545 are merely directed to measurement of magnetic characteristics of metal materials and cannot be utilized for detection of the degree of degradation of the metal materials.